THE SOIL 



39 



caverns and caves it cuts in stratified sandstone and lime- 

 stone deposits. The wind picking up particles of sand, 

 grinds and wears the rocks to powder. The atmosphere 

 with its moisture and gases aids in this process of 

 solution and disintegration. 



Fig. 17. — Rock Weathering. 

 A step in soil formation. — Chamberlain & Salisbury, courtesy Henry Holt 



& Co. 



With the coming of life upon earth, first the bacteria 

 perhaps and then higher plants and animal life, new 

 forces of disintegration came into play. The bacteria 

 were able to feed upon the rock materials, building liv- 

 ing tissue from the solid minerals which they dissolved 

 and assimilated. Plants and then animals took part 

 in the process, exerting in part mechanical or physical 

 force, in part chemical force and in part biological force. 

 With the decay of vegetable and animal matter organic 

 acids were liberated and these assisted in the process. 



